The default bitmap format in HTML5 is image/png; of course, all photos are by default JPG but there is little point in downloading RAW images if you are ultimately going to save them as JPEG.

Most of the programs I use which have a default bitmap format use PNG. Ever since I started using Linux, I have seen a gradual trend away from JPEGs; a quick count shows I have 32 times more PNGs (from icons to images) on my system than JPGs (a miniscule number of those PNGs were generated by me).